1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to control circuits to regulate an output of a power supply.
2. Background
In a typical switched-mode power supply application, the ac-dc power supply receives an input that is between 100 and 240 volts rms (root mean square) from an ordinary ac electrical outlet. Switches in the power supply are switched on and off by a control circuit to provide a regulated output that may be suitable for providing current to, for example, light emitting diodes (LEDs) for illumination. The regulated output is typically a regulated dc current, and the voltage at the LEDs is typically less than 40 volts.
An ac-dc power supply that provides regulated current to LEDs typically must meet requirements for power factor, galvanic isolation, and efficiency, as explained below. Designers are challenged to provide satisfactory solutions at the lowest cost.
The electrical outlet provides an ac voltage that has a waveform conforming to standards of magnitude, frequency, and harmonic content. The current drawn from the outlet, however, is determined by the characteristics of the power supply that receives the ac voltage. In many applications, regulatory agencies set standards for particular characteristics of the current that may be drawn from the ac electrical outlet. For example, a standard may set limits on the magnitudes of specific frequency components of the ac current. In another example, a standard may limit the rms value of the current in accordance with the amount of power that the outlet provides. Power in this context is the rate at which energy is consumed, typically measured in the units of watts.
The general goal of all such standards for the ac current is to reduce the burden on the system that distributes ac power, sometimes called the power grid. Components of the current at frequencies other than the fundamental frequency of the ac voltage, sometimes called harmonic components, do no useful work, but yet the power grid must have the capacity to provide them and it must endure losses associated with them. Harmonic components generally distort the ideal current waveform so that it has a much higher maximum value than is necessary to deliver the required power. If the power grid does not have the capacity to provide the harmonic components, the waveform of the voltage will droop to an unacceptable value at times that are coincident with the peaks of the distorted waveform of the current. The most desirable ac current has a single frequency component that is at the fundamental frequency of the ac voltage. The ideal current will have an rms value that is equal to the value of the power from the outlet divided by the rms value of the voltage. In other words, the product of the rms voltage and the rms current will be equal to the power from the outlet when the current has ideal characteristics.
Power factor is a measure of how closely the ac current approaches the ideal. The power factor is simply the power from the outlet divided by the product of the rms current multiplied by the rms voltage. A power factor of 100% is ideal. Currents that have frequency components other than the fundamental frequency of the ac voltage will yield a power factor less than 100% because such components increase the rms value but they do not contribute to the output power. The fundamental frequency of the ac voltage is typically either 50 Hz or 60 Hz in different regions of the world. By way of example, the fundamental frequency of the ac voltage is nominally 60 Hz in North America and Taiwan, but it is 50 Hz in Europe and China.
Since the power supply that receives the ac voltage determines the characteristics of the ac current, power supplies often use special active circuits at their inputs to maintain a high power factor. Power supplies that use only ordinary passive rectifier circuits at their inputs typically have low power factors that in some examples are less than 50%, whereas a power factor substantially greater than 90% is typically required to meet the standards for input current, such as for example the International Electrotechnical Commission (IEC) standard IED 61000-3-2. Although regulatory agencies in some regions may impose the standards, manufacturers of consumer equipment often voluntarily design their products to meet or to exceed standards for power factor to achieve a competitive advantage. Therefore, ac-dc power supplies for LEDs, for example, typically must include power factor correction.
Safety agencies generally require the power supply to provide galvanic isolation between input and output. Galvanic isolation prevents dc current from flowing between input and output of the power supply. In other words, a high dc voltage applied between an input terminal and an output terminal of a power supply with galvanic isolation will produce no dc current between the input terminal and the output terminal of the power supply. The requirement for galvanic isolation is a complication that contributes to the cost of the power supply.
A power supply with galvanic isolation must maintain an isolation barrier that electrically separates the input from the output. Energy must be transferred across the isolation barrier to provide power to the output, and information in the form of feedback signals in many cases is transferred across the isolation barrier to regulate the output. Galvanic isolation is typically achieved with electromagnetic and electro-optical devices. Electromagnetic devices such as transformers and coupled inductors are generally used to transfer energy between input and output to provide output power, whereas electro-optical devices are generally used to transfer signals between output and input to control the transfer of energy between input and output.
A common solution to provide high power factor for an ac-dc power supply with galvanic isolation uses two stages of power conversion: One stage without galvanic isolation shapes the ac input current to maintain a high power factor, providing an intermediate output to a second stage of power conversion that has galvanic isolation with control circuitry to regulate a final output. The use of more than one stage of power conversion increases the cost and complexity of the system.
Efforts to reduce the cost of the power supply have focused on the elimination of electro-optical devices and their associated circuits. Alternative solutions generally use a single energy transfer element with multiple windings such as, for example, a transformer or, for example, a coupled inductor to provide energy to the output and also to obtain the information necessary to control the output. The lowest cost configuration typically places the control circuit and a high voltage switch on the input side of the isolation barrier. The controller obtains information about the output indirectly from observation of a voltage at a winding of the energy transfer element. The winding that provides the information is also on the input side of the isolation barrier. To reduce cost and complexity further, the controller can also use the same winding of the energy transfer element to provide energy to the controller and also obtain information about the input to the power supply.
The input side of the isolation barrier is sometimes referred to as the primary side, and the output side of the isolation barrier is sometimes referred to as the secondary side. Windings of the energy transfer element that are not galvanically isolated from the primary side are also primary side windings, sometimes called primary referenced windings. A winding on the primary side that is coupled to an input voltage and receives energy from the input voltage is sometimes referred to simply as the primary winding. Other primary referenced windings that deliver energy to circuits on the primary side may have names that describe their principal function, such as for example a bias winding, or for example a sense winding. Windings that are galvanically isolated from the primary side windings are secondary side windings, sometimes called output windings.
While it is quite straightforward to use a winding on the input side of the isolation barrier to obtain information indirectly about a galvanically isolated output voltage, it is a different challenge to obtain information indirectly about a galvanically isolated output current. In many power supply topologies, the measurement of a current in an input winding alone is not sufficient to determine an output current. Conventional solutions for measuring an output current usually include a current to voltage conversion that wastes power and uses costly components to transmit a signal across the isolation barrier. Therefore, conventional solutions are not satisfactory to meet the goals of galvanic isolation with high efficiency and high power factor at low cost in an ac-dc converter.